The overall goal of this application is to understand whether distinctive features of the neonatal immune system, combined with prompt initiation of combination antiretroviral therapy (cART) in perinatally-infected infants, can sufficiently alter the size and distribution of proviral reservoirs to facilitate viral remission, where cART can be stopped without viremic rebound. A major barrier to HIV-1 remission is the early establishment of latent cellular reservoirs that permit lifelong persistence of replication-competent virus. The opportunity to start cART promptly in infants may severely restrict or abort the formation of these long-lived HIV-1 reservoirs. This concept is exemplified in the recent case of the Mississippi Child in whom cART started by 31 hours of age led to HIV-1 remission. We specifically hypothesize that early or very early cART in the context of HIV-1 infection of a predominantly fetal immune system restricts the size, distribution, and replication-competence of the HIV-1 reservoirs in long-lived central memory CD4+ T cells, eventually permitting HIV-1 remission. In three different cohorts of perinatally-infected children spanning the neonatal period through adolescents, including in a planned clinical trial sponsored by the International Maternal Pediatric Adolescent AIDS Clinical Trials (IMPAACT) Network, we will use ultrasensitive molecular, immunology and virus culture assays to: 1) Confirm ongoing decay of and identify where HIV-1 proviral reservoirs reside under long-term effective cART in perinatal HIV-1 infection, 2) identify a virologic and immunologic profile of HIV-1 near-remission that indicates the appropriate timing of cART cessation after very early or early cART initiation, 3) determine if infection of neonatal CD4+ T cells is associated with diminished HIV-1 integration events and replication-deficient genomes that permit clearance of HIV-1 in neonates initiating very early cART and 4) determine whether a predominance of fetal T and/or myeloid cells at birth is associated with and predictive of rapid decay of a restricted viral reservoir. The proposed project will improve scientific knowledge on the long-term virologic and immunologic effects of early/very early HAART for infants who are now likely to survive to young adulthood. It will also provide insights into the early infection events leading up to HIV-1 reservoir formation and whether prompt antiretroviral therapy will lead to HIV-1 remission or cure, thus sparing HIV- 1 infected children a lifetime of therapy. The studies have direct relevance to the research mission of the National Institutes of Health where finding ways to achieve viral remission or cure is a top research priority area.